This invention relates to an icing detector for moving and stationary installations, and more particularly to a detector suited for VTOL aircraft.
Ice forms on the forward facing surfaces of an aircraft or other body when it flies through or otherwise moves relative to clouds of supercooled water droplets. The droplets freeze almost instantaneously upon impact with the aircraft or body, resulting in the formation of ice. For a given velocity and a particular body geometry, the rate of formation is dependent on the liquid water content of the cloud, water droplet diameter and ambient temperature. Conversely, for a given icing condition and body geometry the rate of ice accumulation is a function of relative velocity.
Although ice detectors are known, there exists a need for ice detectors which operate under widely varying ambient air velocities or where the air velocity is insufficient to provide adequate sensitivity. Examples of applications for such detectors include tall structures such as buildings and towers, vessels, stationary gas turbine installations, and VTOL aircraft.
In helicopters, an icing detector is essential if it is to undertake all-weather flying operations owing to the wide range of airspeed at which a helicopter can operate. The most susceptible part of the helicopter to icing is the rotor blade system; this is because the droplet catch efficiency of the rotor blades is high, owing to their comparatively small scale and high velocity, while the catch efficiency on the non-rotating parts of the helicopter is extremely low particularly at low flight velocities for converse reasons. Thus an icing detector is necessary to give the pilot early warning of an icing encounter so that he may take the necessary precautionary action. In addition, if the helicopter is equipped with a rotor blade icing protection system, an ice detector is a necessary component of that system because of the need for immediate activation of the system on entering an icing condition, and for a close control of the allowable ice accretion thickness where the system works on a de-icing principle. To achieve these aims, it is necessary that the ice detector has a high sensitivity, (i.e., capable of detecting small quantities of ice), and a rapid response. The icing rate indication that it provides should relate closely to the rate of rotor blade icing which is affected only in a minor way by flight speed because of the high rotational velocity of the rotor blades.
Most existing types of ice detectors, both for airborne and for static use, rely on the natural relative velocity between the droplet-laden air and the detector sensing surface to deposit supercooled water droplets on that surface. The presence of the water, either in its liquid or its solid phase, on the sensing surface is detected by various means. Because the rate of impingement of the droplets on the surface is a function of the relative velocity, it follows that the response of the device is also a function of the velocity and decreases with decreasing velocity. In addition certain types of detectors require a minimum relative velocity to achieve ice sensing operation.
On helicopters, present means of ice detection employ a standard aircraft ice detector mounted either on the fuselage or in the engine, or other, air inlet duct. These arrangements have not provided satisfactory ice detection or icing rate data suitable for the control of rotor blade icing protection systems because there is no unique correlation between the icing rate as sensed by the ice detector and that experienced by the rotor blades. Additionally, mounting an ice detector in the engine inlet is not desirable because of the danger of engine damage caused by ice shed from the detector. It has been proposed that an ice detector be mounted on the rotor blade itself, thus placing it where the icing measurement is desired; however, this proposal does not find favour because of the complications of slip-rings or other devices required to transmit power and detector signals between rotating and non-rotating parts, and of the greater difficulties in incorporating the detector in the rotor blade structure. Most existing ice detectors are designed to detect the presence or absence of an icing condition, but do not provide a quantitative measure of the severity of the condition.